Compression ignition engines (e.g., diesel engines) are a type of internal combustion engine which use heat of compression to initiate ignition and burn fuel. Compression ignition engines may have a higher thermal efficiency than other internal combustion engines, such as spark ignition engines, for example. However, compression ignition engines may have increased emissions (e.g., nitrogen oxides (NOx), particulates, or the like), as compared to other internal combustion engines. The increased amount of NOx is attributed to diffusion flame combustion. In the diffusion flame, fuel is oxidized in a stoichiometric fashion which produces relatively high local temperatures. The high local temperatures produce increased levels of NOx. Further, the increased particulate emissions are based upon the heterogeneous combustion event in which local equivalence ratios are high and a tendency for particulate formation increases. As such, an exhaust gas treatment system may be coupled to an exhaust passage of the engine in order to reduce emissions.
In some examples, emissions may be reduced by operating the compression ignition engine as a dual fuel engine which operates using two different fuels, for example, a low reactivity fuel (e.g., natural gas) and a high reactivity fuel (e.g., diesel). In such an example, the engine may have two fuel systems for each of the fuels. Further, the exhaust gas treatment system may include a reductant system so that a reductant can be delivered to an exhaust gas treatment device for reduction of NOx, for example. As such, an amount of space occupied by the engine and exhaust gas treatment system may be increased.